Methanation is an important process for upgrading coal and biological materials to useful fuel gases. In coal gasification, methanation is the catalytic conversion of hydrogen and carbon monoxide to methane. Methanation is also used to produce biomethane from organic sources of energy. One method of distributing vast quantities of coal and biomass energy is to gasify the coal to produce synthesis gas (syngas) and then convert the syngas to substitute natural gas (SNG) via methanation (Reid, 1973). Current methanation processes use a nickel (Ni) catalyst which imposes certain operating limitations (FIG. 1) because of its susceptibility to deactivation by surface carbon, high temperature requirements, and poisoning by various sulfur compounds. The stringent process restrictions shown in FIG. 1 require additional steps for successful use of nickel catalysts (Walston, 2007).
A major restriction for nickel catalysts comes from their extreme sensitivity to poisoning by sulfur compounds which are always present in coal-derived synthesis gas. Syngas processed by nickel catalysts must be purified to below 20 ppb sulfur to avoid poisoning of the catalyst even though pipeline natural gas can contain up to 4 ppm hydrogen sulfide. Nickel catalysts can also be irreversibly poisoned by carbon fouling. To avoid carbon fouling the H2/CO ratio has to be adjusted to values greater than three by the water-gas-shift (WGS) reaction. Nickel catalysts are also deactivated by sintering at high temperatures (>450° C.). The methanation reaction is so highly exothermic that a 5 mole percentage reduction in carbon monoxide (CO) concentration due to the methanation reaction results in about a 260° C. (500° F.) increase in reactor temperature. Ni catalysts are so active that it is hard to obtain less than 100% CO conversion even at high space velocities. Therefore, in industrial plants, around 90% of the product gas from the methanation reactor is recycled back to dilute the concentration of CO to less than 5% (mol) in the feed gas. Lurgi designed the Great Plains Synfuels Plant in North Dakota using this conventional methanation technology (Lukes, 2003; Anand, 2007). This is the only existing example of a commercial coal to SNG facility and has been operating since July 1984.
Improvements to the conventional methanation process were made by Haldor Topsoe and Johnson Matthey by developing a high temperature methanation technology. Haldor Topsoe developed a Ni-based, thermally stable methanation catalyst, MCR-2X, for a high temperature methanation process, trade marked as TREMP™ (Total REcycle Methanation Process) (Udengaard, 2006). This catalyst can be operated at higher temperatures than previous Ni catalysts and does not sinter below 700° C. (˜1250° F.). The methanation unit is made up of a series of adiabatic methanators with inter-stage cooling and gas recycle to control the reactor temperature. As a result of the higher temperature tolerance of the catalyst, per pass CO conversion can be increased. This allows for smaller recycle ratio and methanation reactor size resulting in lower CapEX and OpEX than conventional methanation. The higher effluent gas temperature at the reactor outlet can be used to generate superheated, high pressure steam to further improve energy efficiency of the process. This process was demonstrated in a single 1.7 mmscf/d reactor in the 1980's.
Johnson Matthey has developed a similar high temperature methanation technology. Their catalyst is a modified version of their Ni-based pre-reformer catalyst traditionally used in hydrogen and/or ammonia plants. A newer generation of high temperature methanation catalyst, CRG-LH, was formulated in the 1990's, for improved thermal stability. According to Johnson Matthey, this catalyst has been tested on a pilot scale reactor (¾″ diameter, 8 ft long) at 500-600° C. (˜1100° F.), 30-50 bar for over 1000 hours. The results showed acceptable thermal stability. Demonstration of this catalyst on a larger reactor seems necessary to fully evaluate its commercial capability.
Other improvements to the conventional methanation process are combined shift/Methanation (Graboski, 1975), sulfur-tolerant methanation and catalytic steam gasification. Combined shift/methanation technologies such as Conoco's SUPER-METH™ (Kock, 1979; Sudbury, 1980) Parson's RMPROESS™ (Dissinger, 1980; White, 1975; White, 1976), United Catalyst, ICI, and UOP utilize water formed in methanation for water-gas-shift and hence combine the water-gas-shift and methanation reactions into a single reactor. Apart from an acid gas removal unit upstream of the methanation reactor for H2S removal, an additional acid gas removal unit is required downstream of the shift/methanation process to remove CO2. These technologies were piloted in the 1970s but have not proven to be commercially viable due to costs, complexity, scalability or other complications associated with the demands of refinery methanation processes.
The sulfur-tolerant methanation process developed by the Gas Research Institute (GRI) (U.S. Pat. No. 4,491,639; EP0120590; Happel, 1979; Happel, 1981; Happel, 1982; Happel, 1983; Happel, 1985; Happel, 1980; Huang, 1990; Lee, 1987) in the 1970s is shown in FIG. 1. It shows significant improvements over the conventional methanation and combined shift/methanation processes. The sulfur-tolerant methanation process developed by the Gas Research Institute (GRI) uses a molybdenum based (MoS2) sulfur-tolerant catalyst (Happel, 1982). According to the GRI study, the process shows potential savings in steam usage, reduced recycle rate and a smaller acid gas removal unit. This process was piloted extensively by GRI from 1978 to 1985 in an adiabatic reactor system. The reactor was made from 1″ Schedule 80 pipe loaded 4″ deep with ⅛″ cylindrical catalyst pellets. Their GRI-C-525 catalyst ran for 10,000 hours and the GRI-C-600 catalyst ran for 2,300 hours.
When a nickel (Ni) catalyst is used for methanation and Ni catalyst is susceptible to coke formation, sulfur poisoning, and sintering. To solve these problems, a couple of unit operations such as water-gas-shift (WGS) and acid gas removal (AGR) are required before the methanation reactor. Synthesis gas from the gasifier goes through a sulfur-tolerant WGS reactor to adjust the H2/CO ratio to 3:1 (Reaction 1). Then, CO2 and sulfur compounds are removed from the hydrogen rich synthesis gas in an AGR unit before supplying it to a methanation reactor (Reaction 2).
Water-Gas-Shift Reaction:CO+H2O⇄CO2+H2(Exothermic)  (1)
Methanation Reaction:CO+3H2→CH4+H2O(Exothermic)  (2)
Catalytic steam gasification is another methanation technology, which was first piloted by Exxon (Nahas, 1983; Nahas, 2003; Nahas, 2004; Nahas, 1978; Nahas, 1978) in the 1970s (Anand, 2008). Recently, GreatPoint Energy (GPE), a new technology company, has done pilot plant campaigns in a 1.5 ft reactor on catalytic steam gasification and has plans to build a pilot facility in Somerset, Mass. Although it eliminates the need for an air separation plant, reduces the size of the acid gas removal unit and also combines gasification, shift and methanation reactions into a single reactor, many operational issues must be proven in the Somerset pilot runs to determine if this process is economical.
One possible alternative source of hydrocarbons for producing fuels and chemicals is the natural carbon found in plants and animals, such as for example, in the form of carbohydrates. These so-called “natural” carbon resources (or renewable hydrocarbons) are widely available, and remain a target alternative source for the production of hydrocarbons. For example, it is known that carbohydrates and other sugar-based feedstocks can be used to produce ethanol, which has been used in gasohol and other energy applications. However, the use of ethanol in transportation fuels has not proven to be cost effective and may not be achievable on a scale significant to current fuel requirements.
Carbohydrates, however, can also be used to produce fuel range hydrocarbons. Although some upgrading technology has been developed to turn biologically derived materials into useful fuel and chemical feedstocks. Unfortunately, many carbohydrates (e.g., starches) are undesirable as feedstocks due to the costs associated with converting them to a usable form. In addition, many carbohydrates are known to be “difficult” to convert due to their chemical structure, the hydrocarbon product produced is undesirable, or the conversion process results in relatively low yields of desirable products. Among the compounds that are difficult to convert include compounds with low effective hydrogen to carbon ratios, including carbohydrates such as starches, sugars, carboxylic acids and anhydrides, lower glycols, glycerin and other polyols and short chain aldehydes.
Cortright et al. US2008/0216391 teaches processes and reactor systems provided for the conversion of oxygenated hydrocarbons to hydrocarbons, ketones and alcohols useful as liquid fuels, such as gasoline, jet fuel or diesel fuel, and industrial chemicals. Abhari, US2009/0054701A1 relates to a process for converting by products of the manufacture of biodiesel into industrially useful oxygenated products of greater commercial value.
Molybdenum disulfide (MoS2) based catalysts are important industrial catalysts used in the removal of sulfur compounds from crude petroleum by hydrogenolysis. from Chemical Engineering nanostructured crystals of MoS2 (CE, April p. 15), a highly porous form of MoS2 has also been produced by researchers at the University of Illinois at Urbana-Champaign (edlinks.che.com/4819-541). In U.S. Pat. No. 7,435,760, Hertling et al. propose using an alkali doped Cu catalyst, MoS2 catalyst, and Rh based catalysts to convert synthesis gas to higher alcohols. Different procedures and catalysts have been developed to get active MoS2 catalytic sites on the surface of the catalyst (Fujikawa, U.S. Pat. No. 7,361,624). Additionally, NiMoS and CoMoS catalysts are commercially available from CENTINEL® and ASCENT® based on surface deposition of flat catalyst active sites, but this method of synthesis is limited to surfaces or platelets of catalyst activity which is not ideal for all MoS2 catalyzed reactions.
As such, development of an improved catalysts for converting carbohydrates, including “difficult” to convert starches as mentioned above, to hydrocarbon, would be a significant contribution to the arts. In addition, development of a process for converting carbohydrates to hydrocarbons which yields significant quantities of desirable hydrocarbon products such as aromatics and olefins would be a significant contribution to the art.